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Published August 16, 2012 | Published
Journal Article Open

α-pinene photooxidation under controlled chemical conditions – Part 2: SOA yield and composition in low- and high-NOₓ environments


The gas-phase oxidation of α-pinene produces a large amount of secondary organic aerosol (SOA) in the atmosphere. A number of carboxylic acids, organosulfates and nitrooxy organosulfates associated with α-pinene have been found in field samples and some are used as tracers of α-pinene oxidation. α-pinene reacts readily with OH and O₃ in the atmosphere followed by reactions with both HO₂ and NO. Due to the large number of potential reaction pathways, it can be difficult to determine what conditions lead to SOA. To better understand the SOA yield and chemical composition from low- and high-NOₓ OH oxidation of α-pinene, studies were conducted in the Caltech atmospheric chamber under controlled chemical conditions. Experiments used low O₃ concentrations to ensure that OH was the main oxidant and low α-pinene concentrations such that the peroxy radical (RO₂) reacted primarily with either HO₂ under low-NOₓ conditions or NO under high-NOₓ conditions. SOA yield was suppressed under conditions of high-NOₓ. SOA yield under high-NOₓ conditions was greater when ammonium sulfate/sulfuric acid seed particles (highly acidic) were present prior to the onset of growth than when ammonium sulfate seed particles (mildly acidic) were present; this dependence was not observed under low-NOₓ conditions. When aerosol seed particles were introduced after OH oxidation, allowing for later generation species to be exposed to fresh inorganic seed particles, a number of low-NOₓ products partitioned to the highly acidic aerosol. This indicates that the effect of seed acidity and SOA yield might be under-estimated in traditional experiments where aerosol seed particles are introduced prior to oxidation. We also identify the presence of a number of carboxylic acids that are used as tracer compounds of α-pinene oxidation in the field as well as the formation of organosulfates and nitrooxy organosulfates. A number of the carboxylic acids were observed under all conditions, however, pinic and pinonic acid were only observed under low-NOₓ conditions. Evidence is provided for particle-phase sulfate esterification of multi-functional alcohols.

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© 2012 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Published by Copernicus Publications on behalf of the European Geosciences Union. Received: 2 March 2012 – Published in Atmos. Chem. Phys. Discuss.: 4 April 2012. Revised: 26 July 2012 – Accepted: 30 July 2012 – Published: 16 August 2012. This work was supported in part by Department of Energy grant DE-SC0006626 and National Science Foundation grant AGS-1057183. N. Eddingsaas was supported by the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry. C. Loza and L. Yee were supported by National Science Foundation Graduate Research Fellowships.

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